1. Field of the Invention
The present invention relates to a wobble demodulator for detecting and demodulating a wobble signal from an optical recording medium, on which digital information such as address information is recorded by forming a MSK-modulated wobble in a track, thereby demodulating digital information, and a wobble demodulation method.
2. Description of the Related Art
Recently, the density of an optical recording medium is increasing. In general, a track groove is formed previously on a recordable optical recording medium, and information is recorded along the track groove, i.e., on the track groove or in a region (land) sandwiched by the track grooves. The track groove is formed so as to meander in a sine wave form, and information is recorded in synchronization with a dock generated based on a wobble period of the track groove. Furthermore, in order to record information at a predetermined position on a recording surface of an optical recording medium, an address is provided along the track groove. As a method for writing the address, Phase Shift Keying (PSK) (see JP 10(1998)-69646 A, for example) and Frequency Shift Keying (FSK) (see JP 2001-143404 A, for example) are known conventionally.
FIG. 20 is a block diagram showing a configuration of a conventional wobble demodulator 90 for demodulating digital information from a PSK-modulated or FSK-modulated wobble track as described above. In FIG. 20, reference numeral 401 denotes an optical recording medium on which a wobble track is modulated to be formed, and 402 denotes an optical head for irradiating the optical recording medium 401 with a light beam and detecting the amount of light reflected from the optical recording medium 401 to output an electric signal. Reference numeral 403 denotes a wobble signal detector for detecting a demodulated wobble signal from the electric signal. Reference numeral 404 denotes a carrier signal generator for generating a carrier signal based on the wobble signal. Reference numeral 405 denotes a multiplier for multiplying the wobble signal by the carrier signal, 406 denotes an integrator for integrating a multiplied output obtained by the multiplier 406 on the basis of a carrier period, and 407 denotes a decoder for decoding digital information based on the positive/negative sign of an integrated value obtained by the integrator 406 (see JP 2001-126413 A, for example).
In a section where the frequency or the phase is different between the wobble signal and the carrier signal due to the FSK modulation or the PSK modulation, a multiplied output from the multiplier 405 is negative. By allowing the multiplied output to pass through the integrator 406 on a carrier period basis so as to remove a noise component from the multiplied output from the multiplier 405, digital information can be obtained based on the sign of a value output from the integrator 406.
Furthermore, Minimum Shift Keying (MSK) has been proposed as one of the modulation systems of a wobble signal, in addition to the above-mentioned modulation systems. The MSK modulation is one of the types of the FSK modulation with continuous phases, in which a modulation index is 0.5. According to the FSK modulation, the signs of data to be modulated “0” and “1” are associated with two carrier signals having frequencies f1 and f2, whereby modulation is performed. That is, if the data to be modulated is “0”, a sine waveform of the frequency f1 is obtained. If the data to be modulated is “1”, a sine waveform of the frequency f2 is obtained. Furthermore, in the case of the FSK modulation with continuous phases, the phases of two carrier signals are continuous even at a change position of the signs of data to be modulated. According to the FSK modulation, a modulation index m is defined as follows: m=|f1−f2|T, where T is a transmission speed (time of 0.1/shortest sign length) of data to be modulated. The FSK modulation with continuous phases in the case where the modulation index m is 0.5 is called MSK modulation.
FIG. 21 shows a MSK-modulated wobble signal waveform. One of two frequencies used for the MSK modulation is the same as that of a carrier signal, and the other is 1.5 times that of the carrier signal. More specifically, one of the signal waveforms used for the MSK modulation is Cos (ωt) or −Cos (ωt), and the other is Cos (1.5 ωt) or −Cos (1.5 ωt). As shown in FIG. 21, a wobble signal 211 has a waveform of Cos (ωt), Cos (ωt), Cos (1.5 ωt), −Cos (ωt), −Cos (1.5 ωt), Cos (ωt), Cos (ωt) on the basis of a carrier period. In this waveform, a 3-carrier period section having a signal waveform of Cos (1.5 ωt), −Cos (ωt), and −Cos (1.5 ωt) is referred to as a MSK modulation mark 212.
As shown in FIG. 22, address information is recorded by placing the MSK modulation mark 212 at a predetermined position, setting a 56-carrier period T22 to be one bit block. The MSK modulation mark 212 for taking bit synchronization is placed in the leading 0th to 2nd carrier periods. When the data bit of the address information is “1”, the MSK modulation mark 212 is placed in the 12th to 14th carrier periods. When the data bit of the address information is “0”, the MSK modulation mark 212 is placed in the 14th to 16th carrier periods.
In accordance with a technique extended from a conventional example, a wobble demodulator designed for the above-mentioned MSK modulation is configured, for example, as follows.
FIGS. 23A and 23B are timing diagrams of an operation of detecting a MSK modulation mark by a conventional wobble demodulation circuit. As shown in FIG. 23A, in a MSK modulation mark section, the frequency and the phase are different between a carrier signal and a wobble signal. Therefore, a multiplied output thereof is negative, and a S/H value obtained by integrating the multiplied output on the basis of a carrier period, in accordance with a sample-and-hold signal SH output on the basis of a carrier period, also has a negative value. The decoder 407 takes bit synchronization by measuring an output interval of a MSK detection signal output when the S/H value is negative, and decodes digital information.
FIGS. 24A and 24B show waveforms of wobble signals reproduced from wobble tracks modulated by the above-mentioned modulation systems. FIG. 24A shows a PSK-modulated wobble signal waveform, in which a phase is inverted in a PSK modulation portion. FIG. 24B is a MSK-modulated wobble signal waveform that is a kind of the FSK modulation. With respect to the wobble waveform Cos (ωt) in a non-modulated portion, a wobble waveform of Cos (1.5 ωt), −Cos (ωt), −Cos (1.5 ωt) is obtained on the basis of a carrier period in a 3-carrier period in the MSK-modulated portion.
A format has been proposed, in which address information is recorded in accordance with a position where a modulation mark by the above-mentioned PSK modulation or MSK modulation is placed.
FIGS. 25 and 26 show address formats using the MSK modulation. Address information is recorded on the basis of an address word, and the address word is composed of 83 units. The unit represents a synchronization pattern (SYNC) and a data bit in 56 carrier periods, and the address word is divided into a SYNC part of 8 units representing a synchronization position and a data part of 75 units representing an address value.
FIG. 25 shows a configuration of the SYNC part. The SYNC part is composed of 8 units: a monotone unit, a SYNC0 unit, a monotone unit, a SYNC1 unit, a monotone unit, a SYNC2 unit, a monotone unit, and a SYNC3 unit, arranged successively. At the leading position of each unit, a MSK modulation mark is placed, and in the SYNC0 unit, the SYNC1 unit, the SYNC2 unit, and the SYNC3 unit, MSK modulation marks are placed respectively at different positions.
FIG. 26 shows a configuration of the data part. The data part is composed of a monotone unit, a data1 unit, and a data0 unit. In the data1 unit and the data0 unit, MSK modulation marks are placed respectively at different positions. Furthermore, 5 units (1 monotone unit and 4 data1 units or 4 data0 units) represent an address value of 4 bits (1 Nibble). A data part (15 Nibbles) is composed of address data (9 Nibbles) and a parity (6 Nibbles). Because of this, an error can be corrected.
FIG. 27 is a block diagram showing a configuration of a conventional wobble demodulator 90A for reproducing address information from a wobble track based on an arrangement position of modulation marks by the above-mentioned PSK modulation or MSK modulation. In FIG. 27, reference numeral 1601 denotes an optical recording medium on which a wobble track is modulated to be formed, and 1602 denotes an optical head for irradiating the optical recording medium 1601 with a light beam and detecting the amount of light reflected from the optical recording medium 1061 to output an electric signal. Reference numeral 1603 denotes a wobble signal detector for detecting a modulated wobble signal from the electric signal. Reference numeral 1604 denotes a wobble PLL for extracting a carrier signal based on the wobble signal. Reference numeral 1605 denotes a decoder for reproducing address information based on the wobble signal and the carrier signal (See JP 2002-342941 A and JP 2002-352521 A, for example).
The decoder 1605 multiplies the wobble signal by the carrier signal by the multiplier 1606. Then, in a modulated portion detector 1607, the multiplication result is integrated, and based on the sign of its output value, a modulation mark is detected. Furthermore, a modulation mark may be detected based on the number of rising edges and falling edges of a wobble signal on the basis of a period of the carrier signal.
FIGS. 28A and 28B are timing diagrams in the case where a MSK modulation mark is detected by multiplication. As shown in FIG. 28A, in a MSK modulation mark portion, a multiplied output is negative, whereby the position of the MSK modulation mark can be detected.
A SYNC detector 1608 determines the SYNC0 unit/SYNC1 unit/SYNC2 unit/SYNC3 unit based on the modulation mark position to detect a synchronization position. A wobble counter 1609 has its value pre-set in accordance with the synchronization position detected by the SYNC detector 1608, and counts one address word on the basis of a carrier period. A data decoder 1610 determines the data1 unit and the data0 unit in the data part based on the modulation mark detection position with respect to the wobble counter 1609 and demodulates them, and further corrects errors to output address information.
However, in the case where address information and the like are inserted in a wobble signal in accordance with the MSK modulation described above with reference to FIGS. 20 to 23B, a wobble signal is deformed due to a cross talk component of an adjacent track as shown in FIG. 23B. Therefore, in a conventional wobble demodulator, the output position of the MSK detection signal is shifted to and fro, which may shift a bit synchronization position. As a result, an exact recording position cannot be obtained when user data is recorded, and the performance of reproducing an address may be degraded.
Furthermore, according to the conventional system described above with reference to FIGS. 24 to 28B, immediately after a track position for irradiating an optical recording medium with a light beam is changed due to seeking or jumping to an adjacent track, the frequency and the phase of a wobble signal are changed from the previous ones. Therefore, the frequency and the phase of the carrier signal generated by a wobble PLL are not matched with those of the wobble signal. For example, as shown in FIG. 28B, a MSK modulation mark portion is detected in a shifted state or erroneously detected, whereby the SYNC0 unit/SYNC1 unit/SYNC2 unit/SYNC3 unit are erroneously determined and synchronized to a shifted position. Consequently, an address cannot be reproduced. In order to reproduce an exact address in such a state, it is required that, after a carrier signal whose frequency and phase are matched with those of a wobble signal is to be obtained, a shift of a synchronization position is first detected, and thereafter, an exact synchronization position is detected. It takes a long period of time to reproduce address information, which may degrade the access performance to the optical recording medium.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a wobble demodulator and a wobble demodulation method capable of reproducing address information stably and obtaining an exact recording position.